1. Field of the Invention
The present invention relates, in general, to a spacer grid used for placing and supporting a plurality of fuel rods within a nuclear fuel assembly of a light water reactor and, more particularly, to a nuclear fuel spacer grid having both a plurality of arc-shaped main springs and a plurality of bow-shaped sub-springs on its grid strips. The sub-springs having the same radius of curvature as that of the external surface of each fuel rod and effectively supporting the fuel rods when the main springs fail to support the fuel rods, thus accomplishing desired soundness of the fuel rods within the fuel assembly during an expected life span of the rods, the spacer grid also having a plurality of dimple vanes on the grid strips, the dimple vanes being oppositely embossed in an axial direction of the fuel assembly so as to guide the coolant from one cell to neighboring cells of the spacer grid, thus accomplishing a desired coolant mixing effect without creating an excessive pressure drop within the fuel assembly.
2. Description of the Prior Art
In a conventional light water reactor, a plurality of elongated nuclear fuel rods 9 are regularly and parallelly arranged in an assembly 1 having a square cross-section in a way such that, for example, fourteen, fifteen, sixteen or seventeen fuel rods 9 arc regularly arranged along each side of the square cross-section, thus forming a 14.times.14, 15.times.15, 16.times.16, or 17.times.17 array as shown in FIG. 1. In such a nuclear fuel assembly 1, the elongated fuel rods 9 are placed and supported by a plurality of spacer grids 7 and 8. Each of the fuel rods 9 is typically fabricated by containing a fissionable fuel material, or a uranium core 12, within a hermetically sealed elongated zircaloy tube known as the cladding. The above spacer grids 7 and 8 are individually produced by welding a plurality of intersecting grid strips to each other into an egg-crate pattern prior to encircling the periphery of each grid 7, 8 by four perimeter strips. The top and bottom of the fuel assembly 1 are, thereafter, covered with pallets 2 and 3, respectively, and so the assembly 1 is protected from any external loads acting on its top and bottom. The spacer grids 7 and 8 and the pallets 2 and 3 are also integrated into a single structure using a plurality of guide tubes 4. A framework of the fuel assembly 1 is thus fabricated.
In order to provide a high performance nuclear fuel assembly, it is necessary to effectively remove heat from the fuel rods within the assembly. In the prior art, such a removal of heat from the fuel rods is accomplished by making the temperature of coolant flowing around the fuel rods uniformed by mixing the coolant within the fuel assembly. This finally allows the fuel assembly to be free from a partial overheat. In order to mix the coolant within the fuel assembly in the prior art, two structures have been used as follows. In a first structure, a plurality of coolant mixing vanes are attached to the upper portion of each spacer grid, thus creating a turbulent flow or a swirling flow in the coolant passing through each spacer grid and finally mixing the coolant within the fuel assembly. In a second structure, a plurality of coolant mixing ducts are formed within each spacer grid so as to allow the coolant to be mixed together between neighboring cells while flowing upwardly within each spacer grid.
The coolant mixing vanes of the first structure arc inclined relative to the coolant flowing direction at a predetermined angle so as to maximize the coolant mixing effect in addition to reducing pressure drop. However, the conventional coolant mixing vanes increase the turbulent flow, thus undesirably resulting in an increase in pressure drop. In addition, the above coolant mixing vanes allow the fuel rods to be vibrated within the spacer grid, thus undesirably abrading the external surfaces of the fuel rods.
On the other hand, the coolant mixing ducts of the second structure are formed by integrating two grid strips into a single body, with each strip having a coolant mixing channel. In this structure, each duct is arrayed in an axial direction of the fuel assembly at its inlet end, but is bent at its outlet end so as to be inclined relative to the axial direction of the fuel assembly. Therefore, the ducts guide the coolant from one cell into neighboring cells and accomplish a coolant mixing effect within the fuel assembly. However, this structure uses two strips, thus reducing the coolant flowing area and increasing the pressure drop within each spacer grid. In addition, the ratio of the coolant flowing area to the coolant contact length within each duct is higher than that of the coolant flowing passage outside the duct, and so the duct creates a high resistance to the coolant flow. This structure thus finally reduces the coolant flowing velocity within the ducts, and so the structure may fail to accomplish a desired coolant mixing effect.